Waveguide-type optical device

ABSTRACT

In a waveguide-type optical device, two optical waveguides are formed in a substrate of LiNbO 3  or LiTaO 3 . On the substrate and the two waveguides, a blocking layer is formed to block a diffusion of Li ions from the substrate. On the blocking layer, a buffer layer made from SiO 2  is formed. Each of the electrodes, from which operation voltages are supplied, covers each coupling part of the two optical waveguides, respectively, via the blocking layer and the buffer layer.

FIELD OF THE INVENTION

The invention relates to a waveguide-type optical device which performsamplitude modulation of an optical signal and switching of opticalpaths, and more particularly to a waveguide-type optical switch formedon a LiNbO₃ or LiTaO₃ substrate having an electro-optical effect.

BACKGROUND OF THE INVENTION

A conventional waveguide-type optical switch fabricated on a LiNbO₃crystal substrate has the characteristics of low light absorption, lowloss, and high efficiency due to its high electro-optical effect.

The conventional waveguide-type optical switch is disclosed, forexample, in Shoji Yamada et al., "DC Drift Phenomena in LiNbO₃ OpticalWaveguide Devices", Japanese Journal of Applied Physics, Vol. 20, No. 4,April 1981, pp. 733-737.

Referring to FIGS. 1A and 1B, the conventional waveguide-type opticalswitch is described. FIG. 1A is a perspective view of the conventionalwaveguide-type optical switch structure and FIG. 1B is a sectional viewof the structure.

In the optical switch of FIGS. 1A and 1B, a buffer layer 3 is formed ona LiNbO₃, substrate 1, which includes two Ti-diffused optical waveguides2a and 2b. Electrodes 4a and 4b of a metallic material are formed on acoupling portion 5 of each of the optical waveguides 2a and 2b via thebuffer layer 3, respectively.

The buffer layer 3 prevents light propagating along the opticalwaveguides 2a and 2b from being absorbed by the electrodes 4a and 4b,etc. SiO₂ is used as the buffer layer 3 because its refraction index of1.45,. which is smaller than the refractive index of about 2.2 of anLiNbO₃ or LiTaO₃ substrate and because SiO₂ exhibits low lightabsorption. When the buffer layer's refractive index is small as in thecase of SiO₂, it is more feasible to reduce the thickness of the bufferlayer 3 to prevent the absorption of light than in the case of amaterial having a large refractive index.

When switching voltages are applied to the electrodes 4a and 4b, anexternal electric field concentrates on the buffer layer 3 because thedielectric constant of the buffer layer is smaller than that of thesubstrate. Therefore, the electric field in the substrate is relativelysmall. The magnitude of the switching voltages needed for the switchingand the modulation becomes higher for larger thicknesses of the bufferlayer 3.

Since SiO₂ has a small refractive index and extremely low absorption oflight, is no other material for the buffer layer 3 is superior to SiO₂.

However, when SiO₂ is used as the buffer layer 3 and a DC voltage isapplied between electrodes 4a and 4b, Li ions, diffused from thesubstrate into the buffer layer 3 at the fabrication stage, are pulledby the electric field and collected under the electrodes 4a and 4b.Accordingly, an electric field which is counter to the external electricfield is generated between the electrodes 4a and 4b. The magnitude ofthe counter electric field increases as the total moved amount of ionsincreases with time because SiO₂ has a high ion conductivity despite itsrelatively high electric insulating property. If the externally appliedvoltages are kept constant, the resultant electric field applied to theoptical waveguides is reduced by the generation of the counter electricfield, causing a deterioration of the device characteristics. This shiftin an operating voltage point for the switching or modulating operation,known as DC drift becomes a problem when putting the device intopractical use.

SUMMARY OF THE INVENTION

The object of the invention is to provide a waveguide-type opticaldevice capable of stable operation over a long period of time.

According to the invention, there is provided a wave-guide-type opticalswitching device comprising:

a substrate having an electro-optical effect;

first and second optical waveguides formed in the substrate;

a blocking layer formed on the substrate and the two optical waveguides;

a buffer layer, the refractive index of which is smaller than that ofthe substrate formed on the blocking layer, and

first and second electrodes which cover a coupling part of each of thefirst and second optical waveguides, respectively, via the buffer layerand the blocking layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective view of the structure of a conventionalwaveguide-type optical switching device;

Figure 1B shows a sectional view of the conventional waveguide-typeoptical switching device;

FIG. 2 shows a sectional view of the first embodiment of the invention;

FIG. 3 shows a sectional view of the second embodiment of the invention;

FIG. 4 shows a sectional view of the third embodiment of the invention;and

FIG. 5 shows a sectional view of the fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, a first embodiment of the invention is hereinafterdescribed. FIG. 2 shows a sectional view of the first embodiment. In thefollowing description, the same reference numerals show the sameelements with the same function.

As shown in FIG. 2, the first embodiment comprises:

an LiNbO₃ or LiTaO₃ substrate 1 including two optical waveguides 2a and2b;

a Li ion blocking layer 7 which is formed on the substrate 1 includingthe two optical guides and which blocks mixing by diffusion of Li ionsfrom the substrate 1;

a buffer layer 3, which is the same layer as the buffer layer 3 of FIGS.1A and 1B; and

two electrodes 4a and 4b each formed over the two waveguides 2a and 2brespectively via the blocking layer 7 and the buffer layer 3.

As is clear from a comparison of FIGS. 1A and 1B with FIG. 2, the firstembodiment features a blocking layer 7.

For the materials to be used as the blocking layer 7 formed between thesubstrate 1 and the buffer layer 3, there is imposed a condition thatthe Li ion diffusion rate within it is smaller than in the buffer layer3. Suitable materials for the blocking layer 7 include Si, Si₃ N₄, SiON,and MgF₂.

With such structure, even when a heat treatment is applied during theformation of the buffer layer 3, Li ions diffused from the substrate 1are blocked by the blocking layer 7 and cannot reach the buffer layer 3.By adopting the structure, DC drift caused by the formation of thecounter-electric field due to ion migration can be reduced. In addition,the electric field strength in the substrate 1 at the time ofapplication of voltages to the electrodes 4a and 4b is hardly affectedby making the thickness of the Li ion diffusion blocking layer 7 small,so that it is possible to suppress the rise of operating or switchingvoltage to the electrodes 4a and 4b.

Next, referring to FIG. 3, a second embodiment is explained. As shown inFIG. 3, in the second embodiment, the buffer layer 3 of the firstembodiment is divided into a first buffer layer 3a and a second bufferlayer 3b, which cover the optical waveguides 2a and 2b, respectively viathe blocking layer 7. In other words, the buffer layer 3 of the firstembodiment is eliminated in a region between the electrodes 4a and 4b.

By adopting the second embodiment, migration of ions diffused throughthe buffer layer is avoided and DC drift suppression effect increasesrelative to the first embodiment of FIG. 2.

Referring to FIG. 4, a third embodiment of the invention is described.As shown in FIG. 4, in the third embodiment, a phosphosilicate glass(PSG) film 8 is provided on the entire surface of the first and secondbuffer layers 3a and 3b and on the substrate where the first and secondbuffer layers are absent.

Since the PSG film has the effect of gettering Na ions, it can suppressthe migration of the Na ions in the buffer layers 3a and 3b.

The remaining structure of the third embodiment is the same as thestructure of the second embodiment. Therefore, the third embodiment hasthe effect of suppressing the Na ion migration in addition tosuppressing the Li ion migration.

Next, referring to FIG. 5, a fourth embodiment of the invention isdescribed. The only difference between the fourth embodiment of FIG. 5and the third embodiment of FIG. 4 lies in that the PSG film 8 iseliminated in a region between the electrodes 4a and 4b. The separationof the PSG film 8 into a first and second PSG film 8a and 8b eliminatesthe ion migrations in itself and suppresses the DC drift due to themigrations more effectively than the previous three embodiments.

As described above, the present invention reduces the DC drift ofwaveguide type optical devices. The invention is effective not only foroptical devices constituted of a directional coupler as in theembodiments described above, but also for all waveguide-type opticaldevices including the Mach-Zender type and the total reflection typeusing crossed waveguides.

We claim:
 1. A waveguide-type optical device comprising:a substrate ofelectrooptical material; first and second optical waveguides formed inthe substrate; a substrate ion blocking layer formed on said substrateand said first and second optical waveguides; a buffer layer, arefractive index of which is smaller than that of said substrate, formedon said blocking layer; and first and second electrodes which cover acoupling part of each of said first and second optical waveguidesrespectively via said buffer layer and said blocking layer.
 2. Thewaveguide-type optical device as claimed in claim 1, wherein saidsubstrate is made from a material selected from the group consisting ofLiNbO₃ and LiTaO₃.
 3. The waveguide-type optical device as claimed inclaim 1, wherein said blocking layer is made from a material selectedfrom the group consisting of Si, Si₃ N₄, SiON and MgF₂.
 4. Thewaveguide-type optical device as claimed in claim 1, wherein said bufferlayer is made of SiO₂.
 5. The waveguide-type optical device as claimedin claim 1, wherein said buffer layer is separated into a first bufferlayer and a second buffer layer;said buffer layer is absent between saidfirst and second electrodes; said first and second buffer layers coversaid first and second optical waveguides, respectively, via saidblocking layer; and said first and second electrodes are formed on saidfirst and second buffer layer, respectively.
 6. The waveguide-typeoptical device as claimed in claim 5, wherein said device furthercomprises a phosphosilicate glass (PSG) film, which covers said firstbuffer layer, said second buffer layer and said blocking layer,saidfirst electrode covers said first optical waveguide via said PSG film,said first buffer layer and said blocking layer; and said secondelectrode covers said second optical waveguide via said PSG film, saidsecond buffer layer and said blocking layer.
 7. The waveguide-typeoptical device as claimed in claim 6, wherein said PSG film comprises afirst PSG film portion and a second PSG film portion;said PSG film isabsent between said first and second electrodes; said first electrodecovers said first optical waveguide via said first PSG film portion,said first buffer layer and said blocking layer; and said secondelectrode covers said second optical waveguide via said second PSG filmportion said second buffer layer and said blocking layer.